This application is based on an application No. 2000-385759 filed in Japan, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an image forming apparatus for use as a copying machine, a facsimile machine, a printer or the like and, more particularly, to an image forming apparatus having an LED print head for exposure of a photoreceptor surface.
2. Description of Related Art
Some of image forming apparatuses such as copying machines and printers adapted for electrophotographic image formation are known to employ an LED print head for formation of an electrostatic latent image. In such an apparatus, the LED print head is opposed to the surface of a photoreceptor drum, and has an elongate shape extending alongside the axis of the photoreceptor drum. The LED print head includes about 7,000 LEDs arranged in line.
The lighting period of each of the LEDs of the LED print head is controlled on the basis of a lighting clock signal, and is variably set depending on the tone level of image data. As a result, a longer lighting period allows for heavier exposure of the photoreceptor drum surface, whereby a greater amount of toner adheres onto an exposed area of the photoreceptor drum surface for formation of a thick image. On the other hand, a shorter lighting period allows for lighter exposure of the photoreceptor drum surface, whereby a smaller amount of toner adheres onto an exposed area of the photoreceptor drum surface for formation of a thinner image.
The lighting clock signal for controlling the lighting period of each of the LEDs of the conventional LED print head typically has a constant cycle period, so that the lighting period of the LED is merely controlled so as to have a length which is an integral multiple of the cycle period.
However, the amount of the toner caused to adhere onto the exposed area and the LED lighting period (or the exposure period of the photoreceptor drum surface) are not in a linear relationship but in a nonlinear relationship. Even if the exposure period is increased in a linearly proportional relationship to the tone level of the image data, the density of a toner image and the tone level of the image data are not in a linear relationship because an image development characteristic is nonlinear. Therefore, the density of the toner image is slightly inconsistent with the tone level of the image data.
To solve this problem, it is an object of the present invention to provide an image forming apparatus which has an LED print head and ensures image formation with excellent tone reproducibility.
In accordance with the present invention, there is provided an electrophotographic image forming apparatus which includes an LED print head having a multiplicity of LEDs disposed in an array for exposure of a photoreceptor surface, the image forming apparatus comprising: lighting period setting means which predefines a relationship between an image density and a lighting period; and LED driving means which reads lighting periods according to tone levels of inputted image data from the lighting period setting means and lights the respective LEDs of the LED print head for the read lighting periods.
In the present invention, the lighting periods to be employed for lighting the respective LEDs of the LED print head can be set nonlinearly, so that the LEDs can respectively be lighted for optimum lighting periods according to the tone levels of the inputted image data. As a result, the exposure periods can flexibly and properly be correlated with the tone levels of the image data in accordance with the nonlinear characteristic relationship between the tone levels of the image data and the exposure periods. Thus, image formation can be achieved with excellent tone reproducibility.
That is, it is possible to provide a linear characteristic relationship between the image data and the tone of a toner image for improvement of an image quality.
In the image forming apparatus, the relationship between the LED lighting period and the print image density varies depending on intrinsic characteristics of the apparatus, e.g., the characteristics of the photoreceptor drum and condition settings such as settings of an image development potential and an image transfer potential. Therefore, it is preferred to prepare a plurality of lookup tables for various relationships between the number of pulses of a reference clock signal and the tone level, and selectively employ any of the lookup tables.
Further, the lookup tables are preferably rewritable.
With the aforesaid arrangement, the image forming apparatus according to the present invention is excellent in image forming performance, particularly, in tone reproducibility.